Many network services have been proven exploitable and tools to do so, even for the beginner, have become widely available. Even without compromising information, the temporary blackout of a server or network can mean many hours of lost work and missed business opportunities.
Companies can connect to the Internet, and can exchange data via dialup; isdn and leased lines. Furthermore, employees can be offered remote access options. Moreover, every incoming connection is likely to have outgoing connections as well.
No computer network is completely secure. Like any lock, if it is human built, it can be human broken. A small security measure may take out most amateurs that otherwise would cause an annoyance. On the other hand, a major site should not settle for small security measures.
Security is expensive. Dedicated hardware and software has to be purchased, installed, configured and maintained by either hiring, employing or creating expertise. Often changes have to be made to existing infrastructure requiring more hardware or causing downtime.